Hey, thanks for the great program!

I'm seeing a crash when loading data from the Lookup by ISRC option.

Steps:

1. Match an album
2. On the right of the screen, go to the track, then right click > Lookup by... > Lookup by ISRC
3. Choose a release
4. Click Load into Picard
5. Crash  

This is what the log shows:

```

Traceback (most recent call last):
  File "picard/ui/tablebaseddialog.py", line 304, in accept
  File "picard/ui/searchdialog/track.py", line 190, in accept_event
  File "picard/ui/searchdialog/track.py", line 243, in load_selection
  File "picard/ui/searchdialog/track.py", line 201, in _load_selection_non_nat
AttributeError: 'Track' object has no attribute 'parent_item'

```